Changes to fret diagrams to allow some of the changes

Changes to fret diagrams to allow some of the changes
requested by Jose Luis Cruz.

* fret-diagrams.scm: Add new user-definable properties to fret
diagrams
- orientation: normal or landscape
- xo-font-magnification: size of mute and open string symbols
- mute-string: string used to label mute strings
- open-string: string used to label open strings

* define-grob-properties.scm: Add new user-definable properties
 fret
diagrams
- orientation: normal or landscape
- xo-font-magnification: size of mute and open string symbols
- mute-string: string used to label mute strings
- open-string: string used to label open strings